1. Field of the Invention
The invention is directed to a .pi./2 power divider that divides an input signal onto two signal paths with identical amplitude and 90.degree. phase shift that can be preferably employed for the circularly polarized radio-frequency antenna of a nuclear magnetic resonance tomograph.
2. Description of the Prior Art
Devices for producing tomograms of an examination subject, preferably a human body, using the phenomenon of nuclear magnetic resonance are known. These so-called NMR tomography devices contain a fundamental field magnet that aligns the nuclear spins in the human body, gradient coils that generate a spatially different magnetic field, and contain radio-frequency coils for exciting the nuclear spins and for the reception of the signals emitted by the excited nuclear spins. Given utilization of such a radio-frequency excitation and measuring coil, the inductance of the coil is interconnected with a variable capacitor to form an LC resonant circuit, whereby the capacitor arrangement is then tuned according to the desired frequency. The feeder for the coil also contains a variable coupling capacitor for coupling the resonator to a radio-frequency generator, which is preferably an oscillator having a following transmission amplifier.
In the state of equilibrium, the magnetization of the nuclear spins is aligned parallel to the external magnetic field. In order to obtain a useable signal, the magnetization must be turned by 90.degree.. The precession of the nuclear spins after the 90.degree. pulse causes the induction of a signal in a wire loop, known as the free induction decay (FID). Since the nuclear spins precess with a mechanical precession frequency in the magehertz range, they generate an alternating electromagnetic field in the coil. The received radio-frequency signal is supplied to a detector via a decoupling circuit. A directional coupler or a switch that can be used as a transmission and reception diplexer for the supplied and received power can be used as the decoupling circuit. The magnetic and electrical frequency fields cause a radio-frequency absorption in the conductive body tissue of the patient. This results in a reduction of the resonant quality Q of the radio-frequency coil. Further, the electrical properties of body tissue cause a reduction of the resonant frequency when the patient is introduced into the coil. Both effects are thus patient-dependent and must be corrected before every exposure by "tuning and matching" the radio-frequency coil.
For operating radio-frequency coils, particularly circularly polarized antennas, 3 dB/90.degree. hybrids are utilized as transmission and reception diplexer. Given an ideally matched antenna, that signal that the transmitter supplies into port 1 of the directional coupler is divided to the ports 3 and 4 with equal powers; these signals, however, are shifted by 90.degree. in phase relative to one another. No radio-frequency power emerges at the receiver port 2 since the ports 1 and 2 as well as 3 and 4 are decoupled from one another. In the reception case, the reception signal arising at the port 4 again leads the signal arising at the port 3 by 90.degree.. The two signals thus add in-phase at the receiver port 1 (Taschenbuch der Hochfrequenztechnik, 4th ed. Vol. 1, 1986, pages L27-L35).
The 3 dB/90.degree. hybrids can be constructed in line technology or with discrete components and can also be constructed as mixed versions. In these embodiments, the bandwidth employable for the nuclear magnetic resonance tomography is extremely narrow and, for example, is not significantly more than 2%. At low frequencies of, for example, about 10 MHz, broadband directional couplers consisting of coupled lines would have an unfavorably long line length. When, for example, various types of atoms are to be investigated in a nuclear magnetic resonance tomograph, for example, hydrogen, phosphorous or sodium, i.e., when an examination to be undertaken at different resonant frequencies, then an associated 3 dB/90.degree. hybrid is required for each of these frequencies. Interchanging or switching among several such 3 dB/90.degree. hybrids is relatively complex and time-consuming.
Two-phase networks having the property of producing two output voltages shifted by a constant angle relative to one another at two pairs of output posts via a finite frequency interval are also known. Such networks serve the purpose of generating two output signals that exhibit the same level in the frequency range used and exhibit a phase difference, for example 90.degree.. Each of the two, split signal paths contains a quadripole circuit that has a portion of the transmission factor that is independent of the frequency, i.e., what is referred to as an all-pass. In general, each signal path will contain a chain of a plurality of all-passes of the first or second order (Nachrichtentechnik, No. 5, 1957, pages 200-205).
It is also known that a radio-frequency differential transformer can also be constructed with line transformers (NTZ, 1966, No. 9, pages 527-538).
A .pi./2 hybrid is also known that can be used for driving the circularly polarized antenna of a nuclear magnetic resonance tomograph with an operating frequency of about 2.5 through 8.2 MHz. It contains a series circuit of two transformers having different numbers of turns in their primary and secondary windings. A transmitter and a pre-amplifier are connected to those primaries. The transformers are provided with divided secondary windings. All-pass elements having different dimensioning are connected to these secondaries, these all-pass elements having to be driven anti-phase (Magnetic Resonance in Medicine pages 339-353 (1984)).